The OBJECTID value is assigned internally by ArcSDE and cannot be set by FME.
Because of this you basically have to re-read your features to determine which objectid value was assigned each feature.
It's technically possible (but much more complicated) to use two SQLExecutors rather than the regular writer / FeatureWriter and get the objectid without having to re-read the feature, but it entails short-circuiting ArcSDE and writing directly to the Oracle tables. Unless you're a fairly advanced user I wouldn't recommend it, however, as there are some important potential pitfalls you need to take into account. Starting point for the brave: https://support.esri.com/en/technical-article/000009261